Abstract
The paper deals with the impact of subsequent strokes on the backflashover rate (BFR) of HV overhead transmission lines (OHLs), assessed by means of an ATP-EMTP Monte Carlo procedure. The application to a typical 150 kV Italian OHL is discussed, simulating several tower grounding system arrangements. Subsequent strokes parameters are added to the statistical simulation variables: peak current, front time, time-to-half value, lightning polarity, line insulation withstand, lightning location and phase angle of the power frequency voltage. The input data are fed to an ATP-EMTP complete circuit model of the OHL, including line insulation, lightning representation and tower grounding system, the latter simulated by a pi-circuit model able to simulate the effects due to propagation and soil ionization, at modest computational costs. Numerical results evidence a non-negligible BFR increase (in relative terms) due to subsequent strokes: for spatially concentrated grounding systems the BFR increase approximatively vary in inverse proportion with the low frequency grounding resistance, whereas for spatially extended grounding systems the BFR increase depends on the grounding system behavior at high frequencies.
Highlights
Most faults affecting overhead transmission lines (OHLs) at HV and EHV level are caused by lightning strokes to the line itself
In order to show the self-consistency of the Monte Carlo procedure, i.e., the actual convergence within the given number Ntot of lightning strokes, Figure 8 shows the 100 ̈ (NBFO /N) ratio vs. N yielded by the procedure for GS 6 grounding arrangement, for both cases of subsequent strokes disregarded or accounted for (in [32] 100 ̈ (NBFO /N)
The minimum current causing backflashover strictly depends on R@50Hz for concentrated grounding systems, decreasing as R@50Hz increases; for spatially extended grounding systems, the minimum current causing backflashover depends on the their frequency behavior: for instance, GS 5 has a larger R@50Hz than GS 7, but even a larger minimum current causing backflashover, due to its capacitive behavior at high frequency with respect to the inductive behavior of GS 7
Summary
Most faults affecting overhead transmission lines (OHLs) at HV and EHV level are caused by lightning strokes to the line itself (i.e., direct lightning). The authors proposed a simplified (yet accurate) grounding system model [14,15,16,17], able to reproduce the lightning responses, including soil ionization phenomena, at modest computational costs; the model has been subsequently employed in an ATP-EMTP-based. The paper presents a comprehensive ATP-EMTP-based backflashover performance evaluation tool, building up subsequent strokes into the authors’ Monte Carlo procedure [21,22,23]. The latter previously only took into account first strokes, whereas the presented procedure assumes a number of subsequent strokes statistically variable in accordance with data reported in [24].
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